Assessing the Impact of Meteorological History on Subtropical Cloud Fraction

Abstract

This study presents findings from the application of a new Lagrangian method used to evaluate the meteorological sensitivities of subtropical clouds in the northeast Atlantic. Parcel back trajectories are used to account for the influence of previous meteorological conditions on cloud properties, whereas forward trajectories highlight the continued evolution of cloud state. Satellite retrievals from Moderate Resolution Imaging Spectroradiometer (MODIS), Clouds and the Earth?s Radiant Energy System (CERES), Quick Scatterometer (QuikSCAT), and Special Sensor Microwave Imager (SSM/I) provide measurements of cloud properties as well as atmospheric state. These are complemented by meteorological fields from the ECMWF operational analysis model. Observations are composited by cloud fraction, and mean trajectories are used to evaluate differences between each composite. Systematic differences in meteorological conditions are found to extend through the full 144-h trajectories, confirming the need to account for cloud history in assessing impacts on cloud properties. Most striking among these is the observation that strong synoptic-scale divergence is associated with reduced cloud fraction 0?12 h later. Consistent with prior work, the authors find that cloud cover variations correlate best with variations in lower-tropospheric stability (LTS) and SST that are 36 h upwind. In addition, the authors find that free-tropospheric humidity, along-trajectory SST gradient, and surface fluxes all correlate best at lags ranging from 0 to 12 h. Overall, cloud cover appears to be most strongly impacted by variations in surface divergence over short time scales (<12 h) and by factors influencing boundary layer stratification over longer time scales (12?48 h). Notably, in the early part of the trajectories several of the above associations are reversed. In particular, when trajectories computed for small cloud fraction scenes are traced back 72 h, they are found to originate in conditions of weaker surface divergence and stronger surface fluxes relative to those computed for large cloud fraction scenes. Coupled with a drier boundary layer and warmer SSTs, this suggests that a decoupling of the boundary layer precedes cloud dissipation. The authors develop an approximation for the stratification of the boundary layer and find further evidence that stratification plays a role in differentiating between developing and dissipating clouds.